The requirements for exogenous cholesterol by the free-living stages of Nippostrongylus brasiliensis has been studied in axenic culture using various methods of supplying sterol and other lipids. Sterol analogues are also being used to gain insight into the utilization of cholesterol by the organism. Vitamin B12 is also being used as a biochemical marker to analyze systematic relationships of helminths and to re-evaluate previously proposed evolutionary schemes, particularly for the Cestoda. Also studies are being performed on the uptake and concentration of B12 by parasitic helminths.